This invention relates to a developing apparatus applied to the development of the latent image on an image forming body more particularly in an electrophotographic image forming apparatus.
In the case of a electrophotographic image forming apparatus in which one component type developer or two component type developer is used, the thickness of the developer layer on a rotatable developing sleeve must be thin and uniform in order to obtain a good image.
Conventionally, the thickness of a developer layer has been regulated by a fixed regulating plate, but there is a limit in accuracy when the fixed regulating plate is applied to the developing sleeve. As a result, the lower limit of the developer layer thickness is approximately 0.3 mm. Moreover, it has been difficult to obtain a uniform thin developer layer on the developing sleeve. Therefore various kinds of developer layer thickness regulating apparatuses have been invented to obtain an even thick developer layer apart from using a fixed regulating plate. Some examples are explained below.
(a) The developer layer forming apparatus which is described in Japanese Patent Publication Open to Public Inspection No. 43038/1979 discloses a developer layer forming apparatus for one component developer that has a developer layer thickness regulating device consisting of a resilient blade. in which one end is free and the other end of which comes into contact with the developing sleeve with pressure.
(b) The developer layer forming apparatus which is described in Japanese Patent Publication Open to Public Inspection No. 51848/1979 discloses a developer layer forming apparatus that has a resilient blade comprising a resilient metal plate and a soft elastic material in a pile, and the middle part of the soft elastic material comes into contact with the developing sleeve with pressure to regulate the thickness of one component type developer.
(c) The developer layer forming apparatus which is described in Japanese Patent Publication Open to Public Inspection No. 126567/1984 and No. 129879/1984 discloses a developing layer forming apparatus in which an elastic roller rotating intermittently or continuously is pressed on the developint sleeve and the developer thickness is regulated by means of nip created at the point of contact of two rollers. (d) The developer layer forming apparatus which is described in Japanese Patent Publication No. 12627/1985 discloses an apparatus applied to a developing apparatus which uses one component type developer. In the apparatus, a roller comes into contact with a developing sleeve made of an elastic material and the thickness of the developer is regulated.
(e) As an improved apparatus which is applied to a two component type developer, an art is disclosed in Japanese Patent Publication Open to Public Inspection No. 191868/1987, and No. 191869/1987. The disclosed art is a means to form a thin developer layer on a developing sleeve which is appropriate for non-contact development.
A resilient blade supported by a supporting unit comes into contact with the developing sleeve with pressure. The edge of the resilient blade is set to oppose the stream of the developer on the developing sleeve. The thickness of the developer layer comprising magnetic carrier and toner on the surface of the developing sleeve is regulated in this way. By this method, the thickness of the developer is easily kept thinner and more accurate than by the conventional method.
(f) Other prior arts for use with two component type developer are disclosed in Japanese Patent Publication Open to Public No. 189582/1986 and No. 75563/1987. The arts disclosed will be described as follows. A rigid, layer thickness regulating plate is installed in a developing apparatus and a magnetic substance is mounted on the back of the regulating plate. The middle part or the edge of the regulating plate is pressed onto the developing sleeve by the force of the magnetic attraction between the above-mentioned magnet mounted on the back of the regulating blade and a fixed magnet installed in the developing sleeve.
The conventional developer layer forming apparatuses have faults which will be explained as follows.
In cases (a) and (b) mentioned above, the apparatus utilizes the force which is created by bending the developer layer thickness regulating resilient blade, so the force tends to fluctuate and the blade tends to vibrate according to variations in the rotating speed of the developing sleeve, the nip position and the developer layer thickness. Furthermore, because countermeasures are not taken to prevent the vibration of the developer layer thickness regulating resilient blade, it vibrates in resonance to a vibration which occurs in the apparatus. That is the reason why obtaining a developer layer with a uniform thickness by this apparatus is difficult. Particularly in the case of (b) mentioned above, the developer sleeve is pressed by a soft resilient member, so this tendency is greater especially in case (b), and to make the matter worse, the geometrical shape of the nip is subject to influence by variation of the developing sleeve rotating speed, the nip position, and the developer layer thickness. Therefore, the area of the nip varies. Accordingly, the developer thickness tends to be uneven. Other than these problems, in the case of forming a nip, if one of the nip forming materials or both of them consist of a soft resilient material, further problems will be caused, such as the clogging at the nip caused by the developer and the deformation of the soft resilient material caused by abrasion. These inconveniences tend to occur when developers which contain hard materials such as magnetic materials and fluidization agents are used. The apparatus explained in case (c) is better than the apparatus explained in case (a) in terms of obtaining a stable, uniform, and thin developer layer, but the apparatus (c) is inferior to (a) in its efficiency in dissolving of aggregated particles according to the use of the rotating developer layer thickness regulation means, and in eliminating aggregated particles. It causes a problem in which a mass of particles aggregated at the nip is held as it is, and then is pushed through along with the rotation of the rotating body. Because of the problem, satisfactory efficiency can not always be obtained even if an intermittent operation is conducted, and the image quality gets worse, causing dirty marks and stains. A further shortcoming of the apparatus is that the rotating mechanism, the pressure contact mechanism and so on become complicated in order to balance the pressure.
The pressure area of the developer layer thickness regulating plate of the apparatuses (a), (b), (c), and (d) mentioned above is comparatively large in order to keep the developer layer thickness constant. The reason why is that a large pressure area is necessary in a conventional developing apparatus in which the quantity of developer fed to the nip varies from moment to moment.
Case (c) and Case (d) disclose technology relating to a resilient developing sleeve which is appropriate for a non-magnetic one component type developer, wherein the developer layer thickness regulating plate is pressed against it to form a thin developer layer. Since this technology has a shortcoming in that the developing sleeve is deformed permanently or the elastic modulus of the developer layer thickness regulating plate is apt to vary when it is used for a long time, a stable developer layer can not be formed.
The apparatus of Case (e) was developed for use with the two component type developer, but it is not able to maintain constant efficiency over a long term.
In Case (a), (b), (e), and (f), a thin developer layer is formed by pressing a resilient developer layer thickness regulating blade against a metal developing sleeve. So, it has several defects in that the elastic modulus of the resilient blade varies when used for a long time, the resilient blade causes parmanent deforms, and the developer layer thickness regulating blade needs to be often replaced. It has little endurance. Moreover, if the resilient blade is installed in the apparatus only slightly incorrectly, the pressure on the developing sleeve will vary. As a result, it is difficult to stably regulate the developer layer thickness. The reason this type of apparatus has a serious defect is that extreme accuracy is required in mass production.
In Case (f), the apparatus has a function to dissolve the aggregate of developer which is caused by the magnetic attraction between the magnet installed on the back of the developer layer thickness regulating plate and the magnet installed inside the developing sleeve. But the magnets attract through the plate, so the longer the distance between the two magnets is, the more the magnetic attraction decreases. In other words, the distance variation has much influence on the pressure, therefore the effect of pressing the blade against the developer layer is not stable and uniform, and the aggregate of toner or developer passes through, or the predetermined layer thickness can not be obtained because the apparatus gets clogged by the toner or developer. When clogging occurs, white streaks appear on the image. These are the defects of the apparatus of Case (f).
An object of the invention is to solve these problems and to provide a developing apparatus which can stably form a uniform thick developer layer on a developing sleeve and can prevent aggregated developer or toner from moving into the developing zone.